I guess I'm at a loss or don't understand what you are having issue with?
Any remote (example MTH DCS remote, Lionel CAB1L remote, universal Lionchief remote) say you are controlling 2 or 3 trains at the same time. you technically have to switch, you control one train at a time, and the other 2 keep doing what they were last doing while you control the third.
You have 3 buttons on the universal remote corresponding to 3 different memory slot IDs if you will. All 3 are learning slot meaning one at a time, you put ONE Lionel Bluetooth or Lionchief RF protocol equipped loco powered on the track to train or learn each button position. Once you have programmed 3 different locos, one at a time, into the remote, then you can put all 3 on the track, and then just tap the button to highlight which one is active control at the time, and the other 3 go dimmer but still lit showing they are active and still talking to the remote.
Again basically all controls systems I know of today generally work the same way. You typically add one engine at a time initially to get everything programmed. Once programmed, you can control multiple locos- although still mostly one at a time which one is under active control or the current addressed loco.